In microscopes with direct illumination, the illumination light is guided either through the objective, and therefore parallel or at a small angle with respect to the optical axis of the observation optics, or past the objective, and therefore at a larger angle with respect to the optical axis. The latter arrangement is known as oblique illumination. Oblique illumination has the advantage that the illumination light is not reflected back by the object into the observer's eye. In this way, for example in operation microscopes, dazzling of the operator is avoided. Another advantage is that such illumination devices can be produced with a smaller installation height, since it is possible to obviate elements which inject the illumination light into the objective near the axis of the observation optics and correspondingly add a certain height to the device along that axis.
Oblique illumination, however, can lead to shadowing of the object to be illuminated if objects or structure are present above the object plane and slightly offset from the observation optics' axis. This may occur, for example, in certain operation techniques in which a more deeply lying tissue section is accessed through a narrow opening.
In order to resolve this problem, oblique illumination has been proposed with two illumination beam paths on opposite sides of the observation optics' axis, thereby increasing the likelihood that at least one of the beams will illuminate the object without shadow. The two beam paths extend at an angle relative to one another and with respect to the optical axis of the observation optics. EP-A 1 326 116 discloses a microscope with an illumination arrangement which comprises two light sources and two optical deviating systems for deflecting the first and second illumination beam paths, respectively, onto the object from different directions. In order to adapt to different working distances of the objective, the mirrors of one of the deviating systems are partially displaceable in the vertical direction so that their vertical distance from the objective is always kept constant. The other illumination beam path is not adapted, so that the illumination with different working distances has a different brightness and is sometimes not focused. In addition, the system can be produced only by tolerating a comparatively large installation height. Furthermore, the need to coordinate operation of two light sources increases complexity and expense.
DE A 197 28 035 discloses a microscope with observation optics having an objective with a variable working distance and with oblique illumination. The illumination beam path comes from a single light source and is split into two beam sub-paths which are respectively directed laterally from the optical axis of the observation optics onto the object. The illumination beam paths are inclined in the region close to the object as a function of the working distance or the focal length (convergence distance) of the objective, so that the beam axes meet approximately at the focal point of the objective. To this end, various deviating mirrors are provided. The deviating mirrors' inclination is respectively adjusted individually as a function of the working distance since the mirrors have to be rotated by different amounts when there is a change in the working distance. As explained below with reference to FIG. 1, adjustment is needed because stationary deviating mirrors would lead to a countersense angle change in the two sub-beams, so that they no longer meet in the region of the optical axis. In order to adjust the mirror settings, DE A 197 28 035 proposes a coupling device which couples the rotational movements of the mirrors mechanically to the object movement. Owing to the multiplicity of components involved, which need to be adapted, this type of convergence distance adaptation is complex and expensive.